Heat sink



FIG. 1 is a front isometric view of a heat sink showing my new design;

FIG. 2 is a front view of a heat sink;

FIG. 3 is a rear view thereof,

FIG. 4 is a left side view thereof,

FIG. 5 is a right side view thereof;

FIG. 6 is a top plan view thereof; and.

FIG. 7 is a bottom plan view thereof 

The ornamental design for a heat sink, as shown and described. 